Basics of MOS Devices

Metal-Oxide-Semiconductor (MOS) structure is created by superimposing several layers of conducting and insulating materials to form a sandwich-like structure. These structures are manufactured using a series of chemical processing steps involving oxidation of the silicon, selective introduction of dopants, and deposition and etching of metal wires and contacts.

CMOS technology provides two types of transistors (also called devices):

            A) A n-type transistor (nMOS) and

            B) A p-type transistor (pMOS).

                                                                                                                                  

      Fig 1. Physical Structure and Symbols       a) NMOS Device                  b) PMOS Device                                                

Transistor operation is controlled by ‘electric fields’, so the devices are also called Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) or simply FETs. It is always an integrated structure, there are practically no single individual MOS transistors.

A MOS transistor is primarily a switch for digital devices. Ideally, it works as follows:

            If the voltage at the gate electrode is "on", the transistor is "on", too, and current flow between the source and drain electrodes is possible (almost) without losses.

             If the voltage at the gate electrode is "off", the transistor is "off", too, and no current flows between the source and drain electrode.

Fig 2. Transistor Symbol and switch model

The gate of an MOS transistor controls the flow of current between the source and drain. Simplifying this to the extreme allows the MOS transistors to be viewed as simple ON/OFF switches. When the gate of an nMOS transistor is 1, the transistor is ON and there is a conducting path from source to drain. When the gate is low, the nMOS transistors are OFF and almost zero current flows from source to drain. A pMOS transistors are just the opposite, being ON when the gate is low and OFF when the gate is high.

        

I will discuss  more on the operation of MOS devices in a future post as this was an introduction article. There are many more to come in the series including Fin Fets, Four - Terminal Structures and other modern devices for next generation of Chips.

           

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